(a) Field of the Invention
This invention relates to hemming apparatuses for hemming a workpiece made such as of a metal plate.
(b) Description of Related Art
An example of conventionally known hemming apparatuses for hemming a workpiece, such as a door panel for a motor vehicle, is that disclosed in Japanese Patent Publication No. 3598489. The hemming apparatus disclosed in Japanese Patent Publication No. 3598489 includes upper and lower platens arranged opposite to each other and hems a workpiece having a frame, such as a door panel. Specifically, the hemming apparatus is configured to hem an outer periphery bent portion and an inner periphery bent portion of the workpiece formed at the outer periphery and the inner periphery, respectively, of the workpiece.
The lower platen of the above hemming apparatus includes a support table for supporting a workpiece thereon, inner periphery hem punches for hemming an inner periphery bent portion of the workpiece, inner periphery slide cam units supporting the associated inner periphery hem punches to allow them to move in a direction towards and away from the inner periphery bent portion of the workpiece, outer periphery hem punches for hemming an outer periphery bent portion of the workpiece, and outer periphery link cam units supporting the associated outer periphery hem punches to allow them to move in a direction towards and away from the outer periphery bent portion of the workpiece. The inner periphery hem punches and the inner periphery slide cam units are disposed inside the frame of the workpiece supported on the lower platen.
On the other hand, the upper platen includes inner periphery driver cam units configured to engage against associated cam followers of the inner periphery slide cam units to drive the associated inner periphery hem punches, and outer periphery driver cam units configured to engage against associated cam followers of the outer periphery link cam units to drive the associated outer periphery hem punches.
When the upper platen moves down, the inner periphery driver cam units of the upper platen engage against the associated cam followers of the inner periphery slide cam units of the lower platen and the inner periphery hem punches are thereby pressed against the inner periphery bent portion of the workpiece, so that hemming forces act on the inner periphery bent portion. At this time, the outer periphery driver cam units of the upper platen engage against the associated cam followers of the outer periphery link cam units of the lower platen, so that hemming forces likewise act on the outer periphery bent portion of the workpiece. In this manner, the outer periphery bent portion and inner periphery bent portion of the workpiece are hemmed.
In order to hem an inner periphery bent portion of a workpiece having a frame as described above, it will do well to provide the inner periphery hem punches and the inner periphery slide cam units on the lower platen so that they come inside the frame of the workpiece. However, there may be workpieces having small frames. In such a case, it is difficult to arrange the inner periphery slide cam units within the frame of the workpiece. To cope with this, it can be considered that the hemming apparatus includes two lower platens, one of the two lower platens has small first inner periphery hem punches and first inner periphery slide cam units provided to come inside the frame of the workpiece and hem part of the inner periphery bent portion of the workpiece and the other lower platen has small second inner periphery hem punches and second inner periphery slide cam units provided likewise to come inside the frame of the workpiece and hem the remaining part of the inner periphery bent portion of the workpiece. Thus, it is possible that after part of the inner periphery bent portion of the workpiece is hemmed by one lower platen, the workpiece is removed from the one lower platen and supported by the other lower platen and the remaining part of the inner periphery bent portion of the workpiece is hemmed by the other lower platen. In this case, however, the number of process steps increases, thereby increasing the man-hour and in turn causing cost rise. In addition, since two kinds of lower platens are provided, this increases the equipment cost and also causes cost rise. Furthermore, since the workpiece is removed from one lower platen and supported by the other lower platen, a misalignment of the workpiece from the support table inevitably occurs, which invites deteriorated processing precision and in turn may degrade the product quality.
The present invention has been made in view of the foregoing points and, therefore, an object thereof is to allow an inner periphery bent portion of a workpiece having a frame to be hemmed in a single step with a single platen, thereby reducing the cost while giving the product a good quality.